1. Field of the Invention
The invention is in the field of pharmaceutical chemistry. The invention relates in particular to a method of reversing local anesthesia induced by a local anesthetic and an alpha-adrenergic agonist, comprising administering an effective low dose of a stabilized liquid formulation of an alpha-adrenergic antagonist.
2. Related Art
Local anesthesia is widely used by dentists to provide pain relief to patients during dental procedures. To provide pain relief, a drug formulation containing a local anesthetic compound such as lidocaine is injected into the gum tissue surrounding the tooth or teeth on which the dental procedure is to be performed. There are short-acting and long-lasting local anesthetic drug formulations. Short-acting local anesthetic drug formulations contain lidocaine or a related local anesthetic drug dissolved in saline or other suitable injection vehicle. Typically, local anesthesia with short-acting local anesthetics lasts approximately 20–30 minutes, which is not long enough for many dental procedures. To obtain long-lasting local anesthesia, dentists often use lidocaine or other local anesthetic formulations which, in addition to the local anesthetic drug itself, contain low concentrations of epinephrine or another adrenergic receptor agonist such as levonordefrin. More than 90% of the local anesthesia procedures performed by dentists involve local anesthetic formulations containing alpha adrenergic receptor agonists. The vasoconstrictor is necessary because local anesthetics without vasoconstrictor are too short-acting for most dental procedures. The added epinephrine stimulates alpha adrenergic receptors on the blood vessels in the injected tissue. This has the effect of constricting the blood vessels in the tissue. The blood vessel constriction causes the local anesthetic to stay in the tissue much longer, resulting in a large increase in the duration of the anesthetic effect (from 20 minutes for the short-acting formulation to 3–6 hours for the long-lasting formulation). A major problem with the use of epinephrine-containing local anesthetics is soft-tissue anesthesia (lip, cheek, tongue) which usually lasts many hours longer than anesthesia and analgesia of the tooth pulp. Tooth pulp anesthesia and analgesia are the desired effects of local anesthesia from a dental procedural perspective while soft-tissue anesthesia is usually an undesirable side effect. Soft tissue anesthesia results in a number of problems and inconveniences, such as a prolonged and uncomfortable feeling of numbness in and around the mouth, inability to smile, difficulty eating, drinking and swallowing, loss of productivity by missing work hours or meetings etc. Lingering soft-tissue anesthesia can be the cause of injuries due to biting of the tongue or lips. Furthermore, lingering soft-tissue anesthesia is an inconvenience and it is perceived as an annoyance by many patients. Lingering soft-tissue anesthesia can lead to injury especially in children who often bite into the anesthetized tissue out of curiosity. It would therefore be desirable to have a drug that could be used at will by dentists to rapidly reverse local anesthesia after it is no longer needed
U.S. Pat. No. 4,659,714 discloses a method of prolonging local anesthesia by coadministering a vasoconstrictor, in particular, a vasoconstrictor that acts upon the alpha adrenergic receptor sites of the blood vessel walls. The '714 patent also discloses the subsequent administration of an alpha adrenergic receptor antagonist to cause reduction of the prolonged anesthesia effect. Included within the group of alpha adrenergic receptor antagonists described in this patent are phentolamine mesylate. However, the examples make reference to the administration of “phentolamine.” It is much more likely that what was administered was phentolamine mesylate since phentolamine mesylate is FDA approved and readily soluble in water. In contrast, phentolamine is not FDA approved and is relatively insoluble in water.
As shown in Example 1, Table 1 of the '714 patent, 0.5–1.5 mg of “phentolamine” was administered to groups of patients which were pretreated with lignocaine admixed with epinephrine. The results in Table 1 show a reduction in the duration of anesthesia with increasing amounts of “phentolamine.” In Example 2, 2 mg of “phentolamine” was administered. In Example 3, four injections of 1 mg each (4 mg total) of “phentolamine” were administered. In Example 4, four injections of 1 mg each (4 mg total) of “phentolamine” were administered.
The drug doses of “phentolamine” described in the '714 patent (0.5–4 mg) overlap the doses of phentolamine mesylate that are approved by the FDA for the systemic treatment of high blood pressure in patients with pheochromocytoma (total dose of 5 mg in a solution of 2.5–5 mg/ml). Since those doses are normally intended for systemic treatment of high blood pressure, those high dose levels can cause severe side effects when used in healthy, normal people. The package insert of the phentolamine mesylate product states the following side effect warning: “Myocardial infarction, cerebrovascular spasm, and cerebrovascular occlusion have been reported to occur following the administration of phentolamine, usually in association with marked hypotensive episodes.” Thus, the drug doses taught by the '714 patent for the reversal of local anesthesia may cause unacceptable side effects, precluding the use of this product for anesthesia reversal in healthy normal subjects in a dentist's office.
It has been discovered that a highly effective local anesthesia reversal can be obtained by injections of much lower concentrations of phentolamine mesylate than is disclosed in the '714 patent. See WO 01/85171. It has been found that a solution containing only 0.05 mg/ml of phentolamine mesylate can rapidly reverse the effect of a local anesthetic containing an alpha adrenergic receptor agonist. This phentolamine mesylate drug concentration is 20–100 times lower than the phentolamine mesylate drug concentration taught by the '714 patent. The advantage is that, at such low phentolamine mesylate drug concentrations, no systemic side effects such as myocardial infarction and cerebrovascular spasm will be observed. This allows the safe and effective use of phentolamine mesylate for local anesthesia reversal without causing life-threatening or other untoward side effects. Indeed, in a human clinical efficacy study using a low concentration formulation of phentolamine mesylate, a highly effective anesthesia reversal was observed without any side-effects whatsoever. This constitutes a crucial improvement of the local anesthesia reversal method taught by the '714 patent.
The present invention is directed to the discovery that prior art formulations of phentolamine mesylate are unstable in water and can not be stored reconstituted in water or saline. Stable liquid formulations have also been discovered which allow prolonged storage of phentolamine mesylate.